Embodiments are generally related to printing methods and systems. Embodiments are also related to developing tone reproduction curves that facilitate consistent and accurate printing from pitch to pitch on a photoreceptor and/or an intermediate transfer belt and/or other marking element.
Embodiments refer to printing as the art of producing a pattern, such as text and images, on a substrate, such as paper or transparent plastic. A marking engine performs the actual printing by depositing ink, toner, dye, or similar patterning materials on the substrate. For brevity, the word “ink” will be used to represent the full range of patterning materials. In the past, the pattern was introduced to the marking engine in the form of a printing plate or a light lens. Modernly, digital data are commonly used to specify the pattern. The pattern can be a data file stored in a storage device and/or transmitted to the printer via a network, radio transmission, infrared radio transmission, and the like.
A popular marking engine today is the xerographic marking engine used in many digital copiers and printers. In such a marking engine, a photoreceptor whose electrostatic charge varies in response to light is placed between an ink supply and the substrate. In xerographic systems, the ink is typically toner. A laser or bank of light emitting diodes is used to expose the photoreceptor to light to form an image of the pattern to be printed on the photoreceptor. In the simplest, monochromatic xerographic engines, toner is applied to the image to create a toner image on the photoreceptor, which toner image is then fused onto the substrate. In more complex systems, additional colors of toner are applied.
Color systems include Image On Image (IOI) systems and tandem systems. In an 101 system, such as that shown schematically in FIG. 1, the engine 10 includes plural primary colors 11 which deposit their inks on the photoreceptor 13, which includes multiple pitches 14. The single photoreceptor 13, such as a belt, receives the first toner image in a first color, which remains on the photoreceptor 13 while a second toner image is created in a second color atop the first image, the first and second toner images remain on the photoreceptor while a third toner image is created in a third color atop the first and second images, et cetera. Once all of the toner images have been placed on the photoreceptor 13, they are transferred to the substrate, typically paper, and fused to the substrate.
In an embodiment of tandem system architecture, such as that shown in FIG. 2, the marking engine 20 includes multiple primary colors 21 which first deposit their inks on respective photoreceptors 22, typically drums, to form toner images, which are then deposited on the intermediate transfer belt (ITB) 23, which includes multiple pitches 24. Each toner image is transferred onto the ITB before the next toner image is formed. Like the IOI system, the toner images are fused once all for a given pitch have been deposited on the ITB.
In a variant of the tandem system shown in FIG. 2, each ink station can include an additional drum between the photoreceptor and the ITB, an intermediate drum, that accepts the toner image from the photoreceptor drum and deposits it on the ITB. The inclusion of the intermediate drum reduces the likelihood of toner of another color getting into a given ink source due to electrostatic interactions between the toner image on the ITB and the photoreceptor drum. Each of the printing architectures found in the marketplace has advantages, but all suffer from color reproduction problems.
In color science, color spaces are used to describe colors. For example, the Pantone colors are a color space commonly used by graphic artists to identify different colors. Another important color space defined by the CIE is known as L*a*b*, where L*, a*, and b* specify color coordinates. One of the most important properties of L*a*b* is that it is device independent. In other words, a L*a*b* color will theoretically be the same regardless of when or how it is produced and by what particular device it is produced. However, because of the nature of ink and marking, particularly color ink, placing a mixture of ink on substrate that should theoretically produce a particular L*a*b* color does not necessarily produce that particular color. One of the commonly used ways in which the difference between the desired color and the printed one is quantified by its Euclidian distance in color space. If L*0a*0b*0 and L*1a*1b*1 are the L*a*b* of the desired color and of the printed color, then the difference, ΔE, is defined by the following equationΔE=((L1*−L0*)2+(a1*−a0*)+(b1*−b0*))1/2  (1)It should be appreciated that many other color difference equations are also in use, and not disclosed in this application, which consider perceptual aspects of human visual system. Equation (1) above is thus an example color difference expression.
Part of the reason for the discrepancy between desired and obtained colors is that a different color space, CMYK, is commonly used in printing. The letters CMYK refer to the cyan, magenta, yellow, and black inks that color printers typically use and are primary colors in such systems. Mixing these inks produces the other colors that a marking engine can print.
The problem with CMYK is that it is not device independent for various reasons. One such a reason is that the pigments of inks are not naturally balanced, and their equal combination does not produce a neutral gray. Another reason is that different inks from different sources mix differently on different substrates. For example, in one situation, a certain combination of cyan, magenta, and yellow ink will produce a particular shade of gray. In another situation, the combination could produce a greenish gray. Of course, the color space will be different in printers using other or additional primary colors. For example, some printers add Orange and Violet, creating a six-dimensional color space, but the problem of color variance remains.
Interestingly, it has been found that compensating for color variance throughout the color gamut of the color printer can be achieved by adjusting the ink mixture to produce gray level balance. This can be performed by printing one or more test patches based on particular requested gray levels, analyzing the output with a spectrophotometer, and generating a tone reproduction curve (CRC). The TRC is then used to alter the theoretical combination of ink to produce more accurate color with an actual combination.
An example of this method is seen in U.S. patent application Ser. No. 11/097,727, filed 31 Mar. 2005 and entitled, “Online Gray Balance Method with Dynamic Highlight and Shadow Controls,” incorporated by reference above. TRCs are used to map an input value to an output value as seen, for example, in FIG. 9, to adjust ink application levels.
To show how TRCs can be employed to adjust ink application levels, a TRC for one of the color separations is shown in FIG. 9, albeit not to scale. The input axis 401 and the output axis 402 both have saturation values ranging from 0 to 255, with 0 indicating no saturation (no ink on the substrate and 255 indicating complete saturation (as much ink as possible on the substrate). Saturation values between 0 and 255 indicate intermediate amounts of ink are deposited. Without a TRC, a request for 100 yellow based on a desired color representation results in a corresponding amount of ink. As described above, the inclusion of 100 yellow may not yield the desired color, and with a TRC, a request for 100 yellow can be mapped to a different amount of ink that will produce the desired color. In FIG. 9, when 100 units of ink are input 403, the TRC maps the input to an actual value of 107 output 404. The TRC of FIG. 9 thus maps a request for 100 units of ink into a request for 107 units of ink to produce the desired color.
When using cyan, magenta, yellow, and black inks to produce a process gray. TRCs can be used to more accurately produce a desired gray. If, for example, one desires a process gray of 128 cyan, 128 magenta, 128 yellow, and 0 black, but the marking engine used must employ 131 cyan, 127 magenta, and 130 yellow, and 0 black to achieve the desired result, TRCs can adjust the requested amounts so that the marking engine deposits 131 cyan, 127 magenta, 130 yellow, and 0 black, yielding the desired process gray. Preferably, a different TRC is used for each ink that a marking engine uses so that a CMYK marking engine will have four TRCs. TRCs can have different ranges of saturation values, such as 0 to 1, 0 to 100, or 0-255. Regardless of the input range and output range, all TRCs are used to adjust the amount of ink deposited by mapping an input value to an output value.
In the '727 application, a TRC is normally produced by an algorithm that fits a curve to a series of knots, which can be determined from calibration data. Printing a calibration patch pattern yields a target patch pattern. The desired reflectances of the calibration patches and the measured reflectances of target patches can be used as calibration data. The series of knots can also include a highlight knot and a shadow knots so that the TRC functions better in the highlight and shadow regions.
More specifically, the '727 application discloses a system and method for producing TRCs that work well over all saturation values, including highlights and shadows, by supplying data to produce better TRCs for highlights and shadows. A storage device stores a calibration patch pattern and the calibration patch pattern includes at least two calibration patches. A marking engine can produce a target patch pattern by printing the calibration patch pattern.
The system produces a target patch pattern by using a marking engine to print a calibration patch pattern on a substrate. The calibration patch pattern includes at least two calibration patches. Each calibration patch is developable and has a desired reflectance. When the target patch pattern is produced, each calibration patch is printed as a target patch. The system obtains target reflectances by measuring target patches that are in the target patch pattern. At least two target reflectances can be obtained because the target patch pattern has at least two target patches. The system then determines a target highlight value from data that includes an input highlight value, the target reflectances, and the desired reflectances. The calibration data preferably includes at least one target saturation and at least one maximum desired saturation. Target saturation relates to the amount of ink that is deposited on a substrate. The target saturation can be the maximum amount of a particular ink that the marking engine can deposit on the substrate. The particular ink can be black or a primary color such as cyan, magenta, or yellow. Calibration data can be used to produce a tone reproduction curve. The method can be enhanced by allowing a user to select a target saturation for any of the inks, including cyan, magenta, yellow, or black, that a marking engine uses. A calibration patch based on the user selected saturation can be printed to produce a target patch whose target reflectance is obtained by measuring the target patch. The target reflectance can then be included in calibration data used to produce a tone reproduction curve. Obtaining at least two target reflectances for the patch, a processor uses the target reflectances and input highlight value to produce a target highlight value and a tone reproduction curve which is stored on a storage device.
The '727 application disclosure, however, does not deal with the variations seen from pitch to pitch in multiple pitch systems. Because the magnitude of pitch signature changes with time due to various reasons (e.g., by developer aging, IBT aging, differential belt wear, etc.), embodiments disclosed herein include a calibration and control methodology for achieving high quality and consistent color balanced printing for printers with periodic pitch-to-pitch variations. Preferably, calibration methods for single pitches can be employed, such as the method referred to above. Embodiments contemplate having gray balanced TRCs and updating them frequently for each pitch, thus having different TRCs for different pitches. Embodiments use customized TRCs for each pitch during the course of printing to obtain consistency between pages printed on different pitches. Additionally, embodiments can obtain a customized gray balanced CMYK TRCs for each pitch using control based iterative gray balance methods using a reduced patch set with as few as twenty-two patches, which is easy to schedule to gray balance the print engine on a per pitch basis. Typically, the calibration job is performed as a separate job, then a print job or multiple print jobs are performed before another calibration job is performed. However, embodiments contemplate such calibration during run time.